No, no, go not to Lethe […] (1)
The speaker opens the poem with a reference to Lethe, one of the rivers bordering the Underworld in Greek mythology. Any contact with the river Lethe was supposed to make you forget all of your earthly troubles and worries, which might sound pretty great to someone who is melancholy. What could be better than drinking some water that would make you forget your problems? But the water of the River Lethe didn't cheer you up; it just made you forget. And the speaker ain't having that. He wants us to pay attention to our sorrow, not to forget about it or to think about death in order to avoid it.
Nor suffer thy pale forehead to be kiss'd By nightshade […] (3-4)
Again, the speaker warns us that death (by drinking poisonous nightshade) isn't an option when it comes to dealing with melancholy. Interestingly, this is the only place in which the reader's own body is mentioned: "thy pale forehead." Some critics have suggested that Keats might have been thinking about his brother Tom, who died of tuberculosis shortly before Keats wrote this poem—that was the "pale forehead" that John Keats had most recently kissed, and it was his brother's death that was an obvious cause for his melancholy.
And hides the green hill in an April shroud; (14)
The image of the "April shroud" is another example of how life and death are linked. April is a springtime month, and spring means new life and growing things. But a "shroud" is the cloth that gets wrapped around a dead body in a tomb. How's that for a contrast?